The FTP communication to a particular server is failing when going through your Proxy SG or ASG device.
"202 Command not implemented, superfluous at this site" is displayed on the browser and packet/HAR captures.
If we take a packet capture and analyze the TCP streams containing the FTP communication, we can see the FTP communication starts normally, then the ProxySG sends an MDTM request and the server or upstream device ACKs it but immediately forwards the following FTP error "202 Command not implemented, superfluous at this site". The MDTM message seems to be the problem here (as it is what originates the FTP 202 response).
MDTM is part of the FTP Extensions RFC (refer to https://tools.ietf.org/html/rfc3659#section-3), however it is not accepted by all FTP servers.
Note the ProxySG's FTP Proxy needs to send this message and as long as the FTP Proxy is used, the MDTM message will be sent.
It is recommended to reach the FTP administrators / developers to have them add support for the MDTM FTP method, this way the ProxySG can complete the FTP communication without receiving the 202 error from the server.
If this is not possible or a faster solution is required: